For the purpose of improving the function of a glass or ceramic substrate in its intended use, a low-reflection coating for increasing the amount of light to be transmitted or for preventing glare caused by reflection is formed on the surface of the substrate.
Such low-reflection coatings are applied, for example, to glass sheets for use in vehicle glass panes, show-windows, or photoelectric conversion devices. A so-called thin-film solar cell, which is a type of photoelectric conversion device, employs a glass sheet on which is formed a stack of an underlayer film, a transparent conductive film, a photoelectric conversion layer made of amorphous silicon or the like, and a back-side thin-film electrode that are arranged in this order. A low-reflection coating is formed on the principal surface of the glass sheet opposite to the principal surface bearing the stack, i.e., on the principal surface on which sunlight is incident. Such a solar cell having a low-reflection coating formed on the sunlight-incident side allows an increased amount of sunlight to reach the photoelectric conversion layer or solar cell element and thus generates an increased amount of electricity.
The best-used low-reflection coatings are dielectric films produced by vacuum deposition, sputtering, or chemical vapor deposition (CVD). In some cases, a fine particle-containing film containing fine particles such as fine silica particles is used as a low-reflection coating. Such a fine particle-containing film is formed by applying a coating liquid containing fine particles to a transparent substrate by a technique such as dipping, flow coating, or spraying.
For example, JP 2014-032248 A (Patent Literature 1) discloses a cover glass for photoelectric conversion devices that is formed by applying a coating liquid containing fine particles and a binder precursor to a glass sheet having surface asperities by spraying, followed by drying at 400° C. and then by calcining at 610° C. for 8 minutes. The low-reflection coating of this cover glass can yield an increase of at least 2.37% in the average transmittance for light having wavelengths ranging from 380 to 1100 nm.
Furthermore, JP 2013-537873 A (Patent Literature 2) discloses a coated glass substrate produced by depositing a sol containing tetraethoxysilane, aluminum acetylacetonate, and colloidal silica on a glass sheet by dip coating, followed by heat treatment at 680° C. for 180 seconds. The low-reflection coating provided on this glass substrate yields an increase of 2.5% in the average average transmittance for light having wavelengths ranging from 300 to 1100 nm.
JP 2014-015543 A (Patent Literature 3) discloses a silicon substrate with a coating formed by applying a coating composition containing a colloidal silica, tetraalkoxysilane, and aluminum nitrate to the substrate using a spin coater and then by drying the applied coating composition at 100° C. for 1 minute, the colloidal silica having a dispersed particle diameter greater than an average primary particle diameter and having a shape factor and aspect ratio that are larger than 1 by a certain amount or more. This coating has a refractive index of 1.40 or less, although there is no mention of the increase in average light transmittance brought about by this coating.